This all very interesting, and you have answered his question about as well as it can be answered except you have all missed an important part of this discussion.
What ever you might want to do, your body is the limiting factor. That really comes down to how comfortable you are driving. If the cab is rough and noisy, it will wear you down much faster. if the seat is comfortable, much longer days are possible.
Today we are doing a 247 mile day. Tomorrow we are planning to do a 270 mile day. We have done the run as a single 525, but we didn't feel like this time. But then again, I am working on an excursion plan that will involve doing several 550~600 mile days because we can't think of a reason to stop in between. We can do that easily. If we want to do more, the coach is comfortable enough to sleep one while the other drives, but then the driver does not have a navigator and if you are not on blue roads, that matters.
Matt